git-svn: control destruction order to avoid segfault

git-svn: control destruction order to avoid segfault

It seems necessary to control destruction ordering to avoid a
segfault with SVN 1.9.5 when using "git svn branch".  I've also
reported the problem against libsvn-perl to Debian [Bug #888791],
but releasing the SVN::Client instance can be beneficial anyways to
save memory.

ref: https://bugs.debian.org/888791
Tested-by: Todd Zullinger <redacted>
Reported-by: brian m. carlson <redacted>
Signed-off-by: Eric Wong <redacted>
Signed-off-by: Junio C Hamano <redacted>